As operas go, Debussy’s fin-de-siècle masterpiece Pélléas et Mélisande is anything but action-packed. Ranking alongside Wagner’s Tristan (man and woman fall – or already are – in love; later one or both die) and Poulenc’s La Voix humaine (woman makes phone call) at the low-visibility end of the stage-antics spectrum, Pelléas is famously a work in which Very Little happens. Some say it barely qualifies as an opera.

Perhaps, then, the prospect of two and a half hours of unrelenting musical mystification and Gallic shrugging performed ‘in concert’ was simply too much for Proms devotees. Great swathes of the Royal Albert Hall’s upper echelons were empty and even the arena could offer its occupants an upgrade to business-class breathing space. That an under-populated RAH might provide the atmospheric surroundings essential to the opera’s message seemed almost too much to wish for. But, led by Sir John Eliot Gardiner with forces largely imported from his 2010 production at the Paris Opéra-Comique, this Prom was extraordinary by any standards.

Key to its success was the fact that the performance didn’t struggle against the work’s famous lethargy. The gestures towards staging, although minimalist (and uncredited in the programme), had been carefully thought out. The action moved largely between an armchair and a chaise longue – the latter surely a must-have accessory for any psychodrama, even in concert performance ­– with characters appearing according to cues in the libretto rather than stepping out of a dinner-jacketed line-up. And although dinner jackets were indeed in evidence, the cast was dressed in subtle, telling mutations of evening wear: full white tie for über-patriarch Arkel; floor-length sobriety for Geneviève; glamorised rags (no shoes) for Mélisande; black tie for Pelléas, naively proper at the start and then gradually undone; le smoking for bad-boy Golaud. True to the symbolist spirit of a piece whose dialogues seem constantly at cross purposes, and whose main characters are more ciphers than personalities, the singers barely looked at one another, instead directing most of their lines straight into the auditorium. Each soloist nevertheless managed to convey a committed, urgent sense of fatalism – and the effect was utterly compelling.

The singing, too, was generally excellent, and its delivery by an almost entirely Francophone cast produced a rare treat in perfumed diction. Always vocally secure, Laurent Naouri’s Golaud was above all an absorbing, sinister dramatic presence, his hyper-controlled passion truly horrifying. As the unfortunate lovers, the light – at times crystalline – voices of Philip Addis and Karen Vourc’h were perfectly balanced. Vourc’h’s daring pianissimos (at times on the edge of audibility) were only more powerful in contrast to her sudden, brief outburst of song at the start of Act 3, delivered from the back of the stage and projected with great clarity into the cavernous hall, its echoes fleetingly ideal. Elodie Méchain’s Geneviève provided a smooth, dark-roast counterpoint to Mélisande’s airiness, while Dima Bawab’s Yniold was charmingly agile; Nahuel Di Pierro gave solid support as the shepherd and doctor. As Arkel, John Tomlinson provided a formidable portrait of waning authority; although in places the vocal waning was all too real, his tone often remains beautiful, and his closing soliloquy was genuinely moving.

But, as anyone who doubts Pelléas’s operatic credentials will tell you, the piece’s single most important character is the orchestra. Gardiner and his Orchestre Révolutionnaire et Romantique brought Debussy’s score into sharp focus to create a highly textured surface far from the vague washes of colour heard by the opera’s first critics. There was, of course, plenty of fin-de-siècle languor and delicacy; but even the quietest passages remained finely etched. Wind solos – the phrasing always as long as can be – with the timbral peculiarities and unmistakeable patina of ‘period’ instruments emerged from a backdrop both less strident and more expressively flexible than usual. In short, a more gripping performance of Debussy’s magnum opus is difficult to imagine.

Prommers could linger in the French late-nineteenth and early-twentieth centuries by attending the first lunchtime recital of the Proms Chamber Music series, which opened under 24 hours later with Alice Coote. Alongside by her long-time accompanist Julius Drake, Coote programmed a selection of songs by Fauré, Poulenc, Reynaldo Hahn and others – a new departure from her usual outings in English and German song. As in Pelléas, a certain uniformity in style and content could have been problematic for all but diehard mélodie fans, the repertoire’s endlessly wafting fumée and preoccupation with l’amour and la mort providing rather fewer internal contrasts than its equivalent across the Rhine. Once again, as in Pelléas, Coote’s response was one of determined characterisation, avidly describing musical details and exploring a far broader range of tonal colours than we are apt to hear in her habitual operatic and song repertoires.

The approach was a mixed success. There were moments of extreme, effortlessly floated beauty, others of winning musical humour and, throughout, a sense of real emotional commitment from both Coote and Drake. Yet it was Drake’s calmer, more staid – but always graceful – approach to the music that was the greater success. Coote’s constant variations of tone (across the range as well as used expressively), glissandi sometimes over-indulged and concern with giving each utterance unique contours risked losing a sense of longer musical development. This was particularly problematic in the opening selection of Fauré above all, where meandering phrases are de rigueur; the interpretative angle worked much better in the later, turn-of-the-century songs by Hahn, where exquisite lines did indeed emerge, but ones whose fragmentariness is musically in-built. Coote’s infectious enthusiasm for the repertoire was, of course, persuasive in itself; what was lacking at times was the semi-serious Gallic shrug – the particular expressive ambivalence that separates this song repertoire from others.